Add .travis.yml

Adding a control file for Travis-CI.
Travis-CI 5.2 test.
Travis-CI changes.

- set TEST_PHP_EXECUTABLE env var
- don't apt-get libyaml (already installed)
Travis-CI changes.

Try another way to set TEST_PHP_EXECUTABLE.
Travis-CI changes.

- turn default notifications on
- build against 5.2-5.5
- tweak run-tests.php settings
Travis-CI configuration.

Turn off "detailed" logging.
Add Travis-CI testing.

Add testing of compiled extension against php 5.2, 5.3, 5.4 and 5.5 using the
Travis-CI automated build service. The ability to do this was a major driver
for moving the yaml module into git.php.net and the associated github mirror.